The present invention relates to directional couplers and more particularly to four port directional couplers using a pair of transformers or baluns.
Heretofore, relatively broadband, hybrid four port directional couplers have been provided using two identical transformers as shown in FIG. 1. The four ports, designated A, B, C & D are provided by two identical transformers T.sub.1 and T.sub.2 connected substantially as shown in FIG. 1. In this directional coupler, the coupling between an input and a coupled output is determined solely by the turns ratio, N, of the transformers. For high values of coupling coefficient, C, (C greater than 10db) the turns ratio, N, must be a rational number, some coupling values cannot, in principal, be achieved. For example, a value of coupling C = .sqroot.13 can only be approximated and for many coupling values the approximation cannot be realized without an excessive number of turns in the transformers.
Some effort has partially overcome the limitations of the hybrid four port directional coupler by using specially designed reactive circuits coupling the four ports. However, these efforts have resulted in directional couplers with multipolar response characteristics of limited bandwidth. In addition, they do not possess a symmetry with respect to all ports and they are limited to quadrature couplers.
Accordingly, it is an object of the present invention to provide a relatively broadband non-quadrature directional coupler which is substantially free of the above-mentioned limitations of prior directional couplers.
It is a more specific object of the present invention to provide a relatively broadband four port directional coupler for which the coupling factor between ports is not limited by the turns ratio of transformers in the coupler.
It is a further object to provide such a directional coupler which accomplishes the above objects and, in addition, is physically symmetrical.